1. Technical Field
This invention relates to certain polyoxymethylene blends which are characterized as having improved mold shrinkage and also having a useful overall balance of physical properties.
Polyoxymethylene compositions are generally understood to include compositions based on homopolymers of formaldehyde or of cyclic oligomers of formaldehyde, for example trioxane, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligomers of formaldehyde, with oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 weight percent. Compositions based on polyoxymethylene of relatively high molecular weight, i.e., 20,000 to 100,000 are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e.g., compression molding, injection molding, extrusion, blow molding, melt spinning, stamping and thermoforming. Finished products made from such polyoxymethylene compositions possess extremely desirable physical properties, including high stiffness, strength, and solvent resistance.
Polyoxymethylene compositions containing thermoplastic polyurethanes have been recently developed and said compositions possess extraordinary toughness and/or impact resistance, along with also possessing a good overall balance of physical properties, such as stiffness and elongation. However, these polyoxymethylene/thermoplastic polyurethane compositions, as well as polyoxymethylene alone, which possess excellent properties overall, undergo a relatively high shrinkage in the mold during mold processing. It is desirous to decrease the degree of mold shrinkage experienced by the polyoxymethylene/thermoplastic polyurethane composition (and also the polyoxymethylene itself) during mold processing so that a mold can be designed accurately and so that the mold shrinkage of the polyoxymethylene/thermoplastic polyurethane compositions can be matched with that of other polymeric compositions.
A goal of the present invention is to improve and thereby reduce the mold shrinkage that a polyoxymethylene/thermoplastic polyurethane composition undergoes during mold processing. It is further a goal of the present invention to make such an improvement in mold shrinkage while still maintaining a useful balance of the properties, such as stiffness, elongation, and toughness, that are inherent in the polyoxymethylene/thermoplastic polyurethane composition itself. It has been surprisingly found that the mold shrinkage property of a polyoxymethylene/thermoplastic polyurethane composition is significantly improved and thereby reduced when there is blended into the polyoxymethylene/thermoplastic polyurethane composition at least one amorphous thermoplastic polymer. It has further been surprisingly found that the blending of the at least one amorphous thermoplastic into the polyoxymethylene/thermoplastic polyurethane composition results in a ternary polymer blend having a useful balance of the physical properties, as measured by flexural modulus, elongation, and toughness, that are inherent in the polyoxymethylene/thermoplastic polyurethane blend itself. It has also been surprisingly found that the incorporation of certain amorphous thermoplastic polymers into polyoxymethylene itself results in an improvement in the mold shrinkage of said polyoxymethylene.
The ternary blends of the present invention are especially useful as injection molding resins and are particularly useful where it is undesirable for the polymer resin to undergo a relatively high level of shrinkage during mold processing.
2. Background Art
European Patent No. 0117664 discloses polyoxymethylene compositions having extraordinary impact resistance, which compositions consist essentially of
(a) 5-15 weight percent of a least one thermoplastic polyurethane, which polyurethane has a glass transition temperature of lower than 0.degree. C., and PA1 (b) 85-95 weight percent of at least one polyoxymethylene polymer, which polyoxymethylene polymer has a molecular weight of from 20,000 to 100,000, PA1 (a) greater than 15 weight percent and not more than 40 weight percent of at least one thermoplastic polyurethane, which polyurethane has a glass transition temperature of lower than -15.degree. C., and PA1 (b) at least 60 weight percent and less than 85 weight percent of at least one polyoxymethylene polymer, which polyoxymethylene polymer has a molecular weight of from 20,000 to 100,000,
the above-stated percentages being based on the total amount of components (a) and (b) only, the thermoplastic polyurethane being dispersed throughout the polyoxymethylene polymer as discrete particles, and the composition having a Gardner impact value of greater than 9 J.
U.S. Pat. No. 4,804,716 discloses polyoxymethylene compositions having extraordinary toughness, which compositions consist essentially of
the above-stated percentages being based on the total amount of components (a) and (b) only, the thermoplastic polyurethane being dispersed throughout the polyoxymethylene polymer as a separate phase having an average cross-sectional size of not greater than 0.9 microns, and the composition having a notched Izod value of greater than 375 J/m.
The polyoxymethylene compositions disclosed in these two references are the type of compositions that can be improved by the present invention to yield polyoxymethylene blends characterized as having improved and reduced mold shrinkage and a useful balance of physical properties.
U.S. Pat. No. 4,640,949 discloses compositions consisting essentially of polyoxymethylene and a stabilizer blend, said blend being comprised of a thermoplastic polyurethane with a polyamide dispersed therein as a separate phase. The polyamide is a crystalline resin (nylon 6, nylon 6/6) having a melting point greater than the melting point of the polyoxymethylene itself.
U.S. Pat. No. 4,713,414 discloses compositions having improved toughness comprising polyoxymethylene, a reactive titanate, a multiphase composite interpolymer, and optionally, a thermoplastic polyurethane.
EP 156-285-A discloses mixtures having improved cold tenacity consisting of polyoxymethylene, a rubber-elastic graft copolymer, and a polymeric third component, which can be a polyurethane. The rubber-elastic graft copolymer has a Tg of less than -60.degree. C., which is much less than would be the Tg of the matrix phase of any of the amorphous thermoplastic polymers of the present invention.
Japanese Patent Publication 61-171756 discloses compositions for electrical and electronic parts comprised of 5-30% polyoxymethylene, 5-40% thermoplastic polyurethane, and 55-90% ethylenic unsaturated nitrile-diene rubber-aromatic vinyl copolymer. This reference is directed toward modifying the vinyl copolymer compound, as evidenced by the high percentage of the vinyl copolymer required and the low percentage of polyoxymethylene required. Further, the examples teach away from using greater than ten weight percent polyoxymethylene in the composition.
U.S. Pat. No. 4,179,479 discloses compositions comprising a thermoplastic polyurethane, a polyoxymethylene, and 0.5-10% by weight of a processing aid which is an acrylic polymer having a number average molecular weight of at least 500,000. Such an acrylic polymer would not be injection moldable by standard present techniques.
U.S. Pat. No. 4,683,267 discloses compositions comprising polyoxymethylene, an elastomer, and an adipate-carbonate mixed ester.
The above references provide background information on polyoxymethylene/thermoplastic polyurethane binary blends or polyoxymethylene/thermoplastic polyurethane blends containing another component. None of these references disclose the entire and exact particulars of the presently claimed invention.
Provided below, as background information, are references related to blends/compositions of polyoxymethylene and another component. None of these references teach that the mold shrinkage of a polyoxymethylene blend/composition can be improved by the addition thereto of at least one amorphous thermoplastic polymer.
U.S. Pat. No. 4,535,127 discloses a copolymer consisting of an acetal polymer portion and a thermoplastic elastomer polymer portion. EP 253649 discloses a blend consisting of an amorphous or crystalline polymer and an oxymethylene polymer.
The following references all relate specifically to binary blends of polyoxymethylene with styrene-acrylonitrile copolymer: U.S. Pat. No. 3,642,940; U.S. Pat. No. 3,654,219; BE 741,163; J51-039,746; J68-023,329; and U.S. Pat. No. 3,476,832.
The following references all relate specifically to binary blends of polyoxymethylene with polycarbonate: U.S. Pat. No. 4,526,921; U.S. Pat. No. 4,130,604; and U.S. Pat. No. 3,646,159.
U.S. Pat. No. 3,595,937 relates to a binary composition comprised of polyoxymethylene and polyarylate.
The following references all relate specifically to binary blends of polyoxymethylene with acrylonitrile-butadiene-styrene resins or acrylonitrile-ethylene-propylene-styrene resins: U.S. Pat. No. 3,642,940; U.S. Pat. No. 4,296,216; U.S. Pat. No. 4,052,348; J6-0210,666; J5-9176,342; and J5-8045,252.
None of the above references teach the particulars of the present invention, nor do any teach that the mold shrinkage of polyoxymethylene/thermoplastic polyurethane compositions, and polyoxymethylene itself, can be improved by incorporating into said compositions at least one amorphous thermoplastic polymer.